The present invention relates to a method for manufacturing a dense and crack-free ceramic membrane which selectively transports oxygen when it is subjected to an oxygen partial pressure gradient. Such supported film-based membranes, consisting of mixed-conducting oxides, have a range of technological applications including oxygen separation, electrochemical membranes reactors and power generation.
Ceramic membranes consisting of mixed-conducting oxides allow the selective transport of oxygen when subjected to an oxygen partial pressure gradient, and this ability allows the production of 100% pure oxygen as e.g. described in European Patent Application No. 95100243.5 (EP-A-663230), U.S. Pat. No. 5,108,465, U.S. Pat. No. 5,516,359, U.S. Pat. No. 5,447,555 and U.S. Pat. No. 5,240,480. Oxygen, supplied from compressed air for example, is dissociated on the surface of the mixed-conducting membrane, and it becomes incorporated into the bulk of the oxide, in the form of an oxygen anion. The oxygen is able to move within the oxide lattice via oxygen ion vacancies. A pair of oxygen anions at the surface are able to be recombined and be oxidised to molecular oxygen, which desorbs. If a membrane of the mixed-conducting material is subjected to an oxygen partial pressure gradient, oxygen is able to be selectively transported from the high partial pressure side of the membrane to the low partial pressure side.
The oxygen transported through the membrane may be the desired product, or alternatively, it may be used for the production of synthesis gas as described in U.S. Pat. No. 5,356,728, for partial oxidation of hydrocarbons as in European Patent Application No. 90134083.8 (EP-A-438902) and U.S. Pat. No. 5,714,091 or power generation applications as in International Patent Applications Nos. PCT/NO97/00170, PCT/NO97/00171 and PCT/NO97/00172 (Norsk Hydro ASA). The partial pressure gradient across the membrane may be generated by either supplying compressed air to one side of the membrane, or by reducing the oxygen pressure at the other side of the membrane. The latter could be achieved by pumping, if pure oxygen is the desired product, or by exposing one side to the membrane to a gas which has a low oxygen pressure, for example methane, in a partial oxidation reactor.
The flux of oxygen through such a membrane is determined by the ambipolar conductivity of the membrane material, the oxygen partial pressure gradient and the thickness of the membrane. Therefore, the flux of oxygen through the membrane may be increased by reducing the thickness of the membrane. When the thickness of a ceramic membrane is reduced below approximately 100 xcexcm, handling of the membrane becomes difficult because of its mechanical weakness. This limit on thickness is higher if the membrane is to be subjected to a total pressure gradient, rather than only an oxygen partial pressure gradient.
However, membranes consisting of a dense film or coating on a porous substrate may be prepared. The film will act as a functional layer for the selective transport of oxygen and the substrate will provide mechanical strength to the film. The connected porosity of the substrate allows the transport of gas either to or from the membrane.
A layer which is less than 10 xcexcm in thickness is generally referred to as a film, whereas thicker layers are termed coatings. For the purpose of this document it will not be distinguished between these terms. Similarly, it will not be distinguished between the terms substrate and support.
Many techniques have been used to deposit dense films or coatings onto a supporting substrate. Such methods include chemical vapour deposition (CVD) as described by Y. S. Yin in his doctoral thesis entitled xe2x80x9cChemical and Electrochemical Vapour Deposition of Zirconia Solid Solutions in Porous Ceramic Mediaxe2x80x9d, University of Twente, The Netherlands, 1992 physical vapour deposition (PVD), which is a generic term for a range of sputtering techniques as described by Hayashi and co-workers in Electrochemnical Society Proceedings Volume 95-24, pages 221 to 227, and electrostatic spray pyrolysis (ESP) as in International Patent Application WO 97/21848. However, these techniques have a number of disadvantages which may include a high cost of precursors, a slow film growth rate; an ability to only grow very thin films or the need to have a line-of-sight between the substrate being coated and the film source material.
Film deposition techniques that are based on contacting the substrate with a liquid containing the material to be deposited, either in solution or as a dispersion may be attractive. Two distinct processes are referred to as sol-gel techniques. In the first method of deposition, in-situ formation of a sol occurs in the liquid phase, usually through an alkoxide precursor. The sol which has formed is then deposited onto the substrate by spin coating or dipping. In the second sol-gel techniques, the solution containing a polymeric precursor is deposited onto the substrate, and then a further treatment (hydrolysis or thermal) leads to the formation of the film.
The main object of the invention was to arrive at an improved method for manufacturing a dense and crack-free ceramic membrane which has not the disadvantages mentioned above.
The inventor found that these disadvantages were eliminated if the dense and crack-free ceramic membrane was manufactured by the following steps:
a) preparation of a porous ceramic substrate, with an open network of pores. which allows transport of gas,
b) deposition of a film or coating of an oxygen ion conducting material onto the porous ceramic substrate, by contacting the porous substrate with a colloidal dispersion or slip,
c) thermally treating the substrate and film or coating to produce a dense, crack-free membrane on the surface of the porous substrate.